It's common knowledge that wildlife breeds in the spring. When it comes to medium and large mammals, however, common knowledge is often wrong. Mating season peaks in mid-winter for many mammals, and some species actually mate a year in advance of giving birth.

The fisher, for example, is a large member of the weasel family. They mate in early spring, but shortly after fertilization, the tiny embryos remain in the uterus but do not implant on the uterine wall. After 10 to 11 months, these "blastocysts" finally implant and a functional gestation period of 30 to 60 days follows. Two or three pups are born in March or April, almost a full year after mating. "Delayed fertilization" is characteristic of several other members of the weasel family and black bears.

River otters mate in March or April and give birth to two or three pups the following February or March. Embryos do not implant on the uterine wall for up to 10 months, followed by an actual pregnancy of about 50 days.

Black bears mate in June or early July, but embryos do not implant until the female enters hibernation. Two or three cubs are born in January while still in the winter den.

Delayed implantation permits females to meet the energetic demands of pregnancy while food is abundant. And young animals leave their dens just as their foods become relatively easy to find.

The peak of the striped skunk breeding season is underway right now. From late February through early March, skunks roam the countryside searching for mates. This may explain why we see so many dead skunks along country roads this time of year. After a gestation period of about 63 days, four to six kits are born in April or May.

Though small rodents such as mice and voles breed frequently, larger rodents devote more time and energy to each reproductive effort. Gray and fox squirrels, for example, mate in January and after a 44-day gestation period give birth to four or five young. The young do not leave the nest until they are 10 to 12 weeks old. A second litter is often born in July or August. The first litters are usually born in tree cavities, while summer litters are often raised in leaf nests.

Beavers and porcupines enter the world as precocial young -- fully furred, with teeth erupted and eyes open. Beavers, one of the few mammals that mate for life, breed in February and 12 weeks later give birth to four or five young.

Porcupines have a much slower reproductive rate. After mating some time in October or November and a 210-day pregnancy, females give birth to a single porcupette.

Bobcats can breed throughout the year but most mating takes place in late winter. After a gestation period of 60 days, two to four kittens are born in the spring. Raccoons mate in January or February, and after a pregnancy of 63 days give birth to three to six pups.

White-tailed deer roam widely in search of mates from October through December during the rut. Gestation lasts 200 days and twins are typical when fawns are born in May or June.

And then there's the opossum, North America's only marsupial. They often mate two times a year, once in February and again in early summer. Just 13 days after mating as many as 13 honey bee-sized young are born. Litter size is limited by the 13 teats females have in their pouch. The tiny newborns are naked and blind, but they have well developed forelimbs with sharp claws. They scramble up the female's belly and into her pouch, where they attach themselves to one of her nipples. The tip of the nipple then swells and essentially locks each baby to the teat, where it remains for about 60 days. So though a 'possum pregnancy lasts just 13 days, it essentially continues in the external pouch for two more months.

Though hardly a comprehensive review of mammalian reproductive biology, this brief summary illustrates the diversity of strategies that larger mammals use to get a head start on the mating season.

Send questions and comments to Dr. Scott Shalaway, 2222 Fish Ridge Road, Cameron, WV 26033 or email sshala...@aol.com.